The production of bacteriophage tRNA PRO and tRNA SER represents the best understood example of a biosynthetic pathway leading from DNA to tRNA. The seven terminal steps involved in the biosynthesis of these tRNA molecules have been defined by mutant methodology and confirmed by a reconstruction of the reactions In Vitro from purified precursor RNA intermediates in enzymes. In this work, we have gained some appreciation of the interactions between a precursor RNA and its respective processing and modifying enzymes. The next phase of this work will be directed at obtaining a more comprehensive understanding of the molecular features underlying the specificity of the interactions. This effort is likely to be assisted by the availability of mutants altered in specific interactions, together with a knowledge of the three dimensional structure of the relevant parts of the precursor RNA molecule. An additional effort will be made to extend the biosynthetic back toward the DNA molecule to include a definition of the terminal steps responsible for nucleotide modifications and possible cleavage reactions that lead to the production of the precursor RNA thus far identified.